The present invention disclosed herein relates to a dye-sensitized solar cell and a method of fabricating the same, and more particularly, to a dye-sensitized solar cell including an electrode structure having a conductor with pores regularly formed, which is fabricated by the use of a template, and a method of fabricating the dye-sensitized solar cell.
The present invention has been derived from research undertaken as a part of IT R & D program of the Ministry of Information and Communication and Institution of Information Technology Association (MIC/IITA) [2006-S-006-02], Components/Module technology for ubiquitous terminals.
A dye-sensitized solar cell includes a dye molecule capable of receiving incident light with a visible wavelength to form electron-hole pairs, a semiconductor oxide capable of receiving excited electrons, and an electrolyte reacting with the electrons after working and returning to the solar cell, which significantly differs from a compound solar cell or a wafer type silicon solar cell using p-n junction.
A dye-sensitized solar cell, which has been well known hitherto, was published by Michael Gratzel et al. (refer to U.S. Pat. No. 4,927,721). A photoelectrochemical solar cell published by Michael Gratzel et al. includes a photosensitive dye molecule capable of absorbing a visible light to generate electron-hole pairs, an electrode structure which is a semiconductor oxide formed of nanoparticle titanium oxide (TiO2) with dye molecules adsorbed, an opposite electrode coated with platinum (Pt) or carbon (C), and an electrolyte filled between the semiconductor oxide and the opposite electrode. Because such the photoelectrochemical solar cell can be fabricated with low fabrication cost per electrical power in comparison with the wafer type silicon solar cell that utilizes a p-n junction, so the photoelectrochemical solar cell is being in the limelight recently.
FIG. 1 is a partial sectional view illustrating a flow of electrons created by sunlight in a conventional dye-sensitized solar cell.
Referring to FIG. 1, dye molecules excited by light emitted from the sun eject electrons (e−) into a conduction band of a semiconductor oxide layer 20 formed of nanoparticle titanium oxide. The electrons ejected into the conduction band pass through the nanoparticle titanium oxide and arrive at a conductive substrate 10 formed of a glass coated with fluorine doped SnO2 (FTO). Thereafter, the electrons are transferred to an external circuit (not shown). The electrons, which come back after performing an electrical work in the external circuit, are injected into the semiconductor oxide layer 20 formed of nanoparticle semiconductor oxide through an opposite electrode (not shown), e.g., platinum or carbon electrode, by means of the electron transfer function of an oxidation/reduction electrolyte 30. Finally, the electrons reduce the dye molecule 24 deficient in electrons. In this manner, the conventional dye-sensitized solar cell is operated.
However, before the electrons injected into the semiconductor oxide layer 20 made of nanoparticle titanium oxide from the dye molecule 24 are transferred to the external circuit and perform an electrical work therein, some of the electrons injected into the conduction band stay in an unoccupied surface energy level of the semiconductor oxide layer 20 on which the dye molecule 24 is not adsorbed while passing through the semiconductor oxide layer 20 and the conductive substrate 10. At this time the electron and the electrolyte 30 are recombined so that the electrons do not circulate in a circuit but vanished ineffectively. Accordingly, there is a loss in photovoltaic energy conversion efficiency.
Furthermore, in the dye-sensitized solar cell proposed by Michael Gratzel et al. where the semiconductor oxide layer is used as the electrode structure, a moving passage through which the electrons injected from the dye molecules to the semiconductor oxide layer move to the conductive substrate is also made of the nanoparticle titanium oxide. Accordingly, the electron encounters a strong electrical resistance while the electron moves to a 3-dimensional structured semiconductor oxide layer, leading to a decrease in short-circuit current density (Jsc). Resultingly, since the photovoltaic energy conversion efficiency is determined by multiplication of a current, a voltage and a fill factor of the solar cell, the current, the voltage and the fill factor should be improved to increase the photovoltaic energy conversion efficiency. Particularly, to increase the voltage significantly, there is a method of increasing the electron density of the nanoparticle semiconductor oxide by minimizing the recombination with the electrons.
Example of a conventional method of minimizing the decrease of the photovoltaic energy conversion efficiency occurring in the semiconductor oxide, e.g., the titanium oxide, is as followings. In the conventional method, the semiconductor oxide formed of the titanium oxide is used as the electrode structure, and coated with a semiconductor oxide material having high band gap energy, for example, niobium oxide (Nb2O5) to form an energy barrier between the semiconductor oxide layer and the electrolyte, thus preventing the recombination. According to the conventional method, the photovoltaic energy conversion efficiency was somewhat enhanced. However, because the titanium oxide is basically used as a material for the electrode structure, there is a limitation in electron movement. Therefore, an increase in the photovoltaic energy conversion efficiency was limited. Adsorptive properties of the dye molecule on the niobium oxide are poorer than that of the dye molecule on the titanium oxide.
As another conventional method, there has been an attempt to employ a nanowire-shaped semiconductor oxide as the electrode structure. However, unlike the expectation that the nanowire-shaped semiconductor oxide could increase the photovoltaic energy conversion efficiency because the nanowire itself is a single crystal and thus advantageous for electron diffusion, the electrode structure made of nanowire-, nanorod- and nanotube-shaped semiconductor oxide exhibits poorer photovoltaic energy conversion efficiency compared to the case of using the nanoparticle semiconductor oxide as the electrode structure. Possibly, this is ascribed to the fact that the electrode structure made of the nanowire-, nanorod- and nanotube-shaped semiconductor oxide has a smaller surface area than the nanoparticle semiconductor oxide. In addition to the aforesaid conventional methods, there have been attempts to form the electrode structure using a semiconductor oxide of zinc oxide (ZnO) or tin oxide instead of the titanium oxide. However, the solar cell achieved by this conventional method still exhibits a poorer photovoltaic energy conversion efficiency compared to the solar cell using the titanium oxide for the electrode structure.